Elsevier

Surface Science

Volume 461, Issues 1–3, 1 August 2000, Pages 23-30
Surface Science

Surface phase diagrams for the Ag–Ge(111) and Au–Si(111) systems

https://doi.org/10.1016/S0039-6028(00)00491-XGet rights and content

Abstract

Based on results from recent structural studies and an overview of the literature, we propose surface phase diagrams for the Au–Si(111) system in the supermonolayer regime and for the Ag–Ge(111) system in the submonolayer region. In addition, time–temperature–transformation (T–T–T) curves are proposed to represent the metastable structures present in surface phase systems.

Introduction

Surface phase diagrams provide fundamental understanding of surface and interface phenomena, and can be used to predict the effects of changing parameters such as temperature or metal coverage on the atomic arrangement of surface structures. They also provide the starting point for understanding the kinetics of phase transitions at surfaces. However, to date there have been relatively few studies of the phase diagrams for simple metals on semiconductors where the basic Gibbs phase rules have been applied; instead, ‘phase maps’ summarizing the conditions of temperature and coverage have been used. In many cases these phase maps necessarily contain errors; for instance, unless a particular phase contains sites with variable occupancy, i.e. is a ‘surface solution’, almost without exception it can only occur in combination with another, in what is called two-phase coexistence.

The thermodynamic phase diagram of a particular system is what it will achieve if held for very long times at the specified temperatures. However, kinetic effects must also be considered. The analysis of experimental results for a certain system can be complicated by the presence of metastable phases, due mostly to kinetic effects, or by incomplete phase transitions. We propose here the use of a set of curves commonly referred to in physical metallurgy as time–temperature–transformation (T–T–T) curves as a way to represent the metastable structures that may be present in surface phase systems [1]. The T–T–T curves provide information on the time that elapses, at any selected temperature, before the transformation begins and until it is finished. When an alloy is continuously and slowly cooled, most of the transformation occurs at high temperature, whereas at a fast continuous cooling rate most of the transformation occurs at low temperatures. The ‘start’ and ‘finish’ lines are usually defined as the moments when 1% of the parent phase has transformed and when 99% of the transformation has occurred respectively.

Based on the atomic arrangement of all phases present and the interphase relationships, we report here a survey of the results of all relevant experimental reports and tentative surface phase diagrams for the Ag–Ge(111) system in the submonolayer regime and the Au–Si(111) system in the supermonolayer regime.

Section snippets

Background for Au–Si (111) and Ag–Ge (111)

An overview of the literature on thin metal deposits on clean semiconductor surfaces, in particular for the Au–Si(111) and Ag–Ge(111) systems, shows a great difference in the number of studies. Although the Au–Si(111) interface is one of the most extensively investigated, for the Ag–Ge(111) system the available information is, at best, fragmentary. A complete understanding of surface phenomena in metal–semiconductor systems requires knowledge of the atomic arrangement. Recently, several reports

The Ag–Ge(111) system

When Ag is deposited on the Ge(111) native reconstruction [the Ge(111)-c(2×8) surface], the interface undergoes a (3×1) Ag, a (4×4) Ag, and eventually a (3×3)R30° Ag reconstruction at a substrate temperature of between 200 and 450°C in the submonolayer regime. For Ag coverages above 0.1 ML, a (4×4) phase is observed, initially coexisting with the c(2×8) surface. This phase was first reported to be complete at 0.27 ML [32] and later at 0.375 ML [6], [10]. Hammar et al. [33] identified for the first

The Au–Si(111) system

The groundwork for extending the phase diagram of the Au–Si(111) system to the supermonolayer regime (up to 1.5 ML) has been laid out by the work of Plass [12]. Fig. 1 shows all the results pertaining to this problem. The experimental observations mapped in Fig. 1 are used as the basis of this study. Plass [12] did not attempt to extend his phase diagram representation to the supermonolayer regime since crucial pieces of information, particularly the atomic geometry of the β(3×3) and (6×6)

Discussion

In this study, we have applied the long-standing principles of physical metallurgy to the study of surface phases. The use of T–T–T curves to explain kinetically constrained phases and represent the evolution of the phase transformation with a family of curves showing different percentages of completion appears to be beneficial, although more work is required to determine the details. Since the surface phases seem to obey the same general principles as their bulk counterparts, the introduction

Acknowledgements

This work was supported by the National Science Foundation on grant number DMR-9214505.

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